Nguyen Thanh-Vinh

MEMS two-axis force plate array used to measure the ground reaction forces during the running motion of an ant

A terrestrial insect can perform agile running maneuvers. However, the balance of ground reaction forces (GRFs) between each leg in an insect have remained poorly characterized. In this report, we present a micro force plate array for the simultaneous measurement of the anterior and vertical components of GRFs of multiple legs during the running motion of an ant. The proposed force plate, which consists of a 2000??m???980??m???20??m plate base as the contact surface of an ants leg, and the supported beams with piezoresistors on the sidewall and surface are sufficiently compact to be adjacently arrayed along the anterior direction. Eight plates arrayed in parallel were fabricated on the same silicon-on-insulator substrate to narrow the gap between each plate to 20??m. We compartmented the plate surface into 32 blocks and evaluated the sensitivities to two-axis forces in each block so that the exerted forces could be detected wherever a leg came into contact. The force resolutions in both directions were under 1??N within ?20??N. Using the fabricated force plate array, we achieved a simultaneous measurement of the GRFs of three legs on one side while an ant was running.

Micro suction cup array for wet/dry adhesion

We propose a micro suction cup array for adhesion to both dry and wet surfaces. We measured the peel-off forces of a PDMS micro suction cup array, a PDMS flat-tip micro structure array and a flat PDMS pad from both wet and dry glass surfaces. When the glass surface was wet, the peel-off forces of a PDMS flat-tip micro pattern and a flat PDMS pad decreased by more than 1.7 times and 7.0 times, respectively. On the other hand, peel-off forces of a PDMS micro suction cup array increased by over 1.1 times when the glass surface was wet. Also, in both wet and dry conditions, an array of PDMS micro suction cup adhered stronger to a glass surface than a PDMS flat-tip micro pattern and a flat PDMS pad. Furthermore, we demonstrated that the adhesion of a suction cup array can be enhanced by miniaturizing the size of the cups.

A viscometer based on vibration of droplets on a piezoresistive cantilever array

This paper reports a method to measure viscosity based on the resonant vibration of droplets on a piezoresistive cantilever array. We demonstrate that viscosity of small droplets (3μL) can be estimated from the attenuation rate of the cantilever output during free-decay of the droplet vibration. The estimation for liquid sample with different viscosity using water-glycerol solutions were carried out.

Interaction forces during the sliding of a water droplet on a textured surface

We have directly measured the interaction forces (in normal and lateral directions) during the sliding of a water droplet on a surface decorated with a micropillar array. The measurement was carried out using a MEMS-based two-axis force sensor array. The advantages of our sensor array were the high sensitivity and miniaturized size of the ultrathin cross-shaped piezo-resistive silicon structure fabricated under a micropillar to detect both the normal and lateral forces acting on the micropillar. A demonstrating measurement using a water droplet with volume of 18 μL sliding on the micropillar array was carried out. The measurement results showed a fluctuation in the interaction forces when the micropillar was close to the trailing edge or leading edge of the droplet. Meanwhile, in the inner region of the contact line, both the normal and lateral interaction forces were relatively stable. These results indicate that the interaction forces at the edges of the droplet are important factors controlling the sliding motion of the droplet.

Measuring the vibration of cells subjected to ultrasound using a MEMS-based force sensor array

This paper reports on a method to measure vibration occurring on the membrane of a mammalian cell which is subjected to ultrasound. The measurement is based on an array of piezoresistive MEMS force sensors. Experimental results with the fabricated sensors showed a change in frequency response to the ultrasound-induced vibration of NIH3T3 cells adhered to the fabricated sensor.

High sensitive 3D tactile sensor with the structure of elastic pyramids on piezoresistive cantilevers

We propose a highly sensitive three-dimensional tactile sensor using the structure of elastic micro pyramids pressing on piezoresistive cantilevers. In the structure of the sensor we proposed, the forces acting on the surface of the elastomer were transmitted to the cantilevers through the pyramids. The key point of our sensor was that the cantilevers were not completely embedded inside the elastomer: a cavity under each cantilever enabled the larger deformation and thus the larger resistance change of the cantilever. Therefore the high sensitivity of the sensor could be obtained. Moreover, by using four cantilevers aligned with four pyramids, the three-dimensional force sensor was realized. The sensitivities of our sensor to forces in normal and lateral directions were about 50 times and 2.4 times higher, respectively, compared to those of a tactile sensor with the ultrathin cantilevers embedded inside an elastomer [1].

Pressure distribution on the contact area during the impact of a droplet on a textured surface

In this paper, we report on a method to directly measure the pressure distribution on the contact area during the impact of a droplet on a textured surface. By fabricating an array of MEMS-based force sensors underneath the microstructures of the surface, the normal forces acting on the microstructures when the droplet comes into contact with the surface can be detected. We show that the pressure becomes maximum in the center of the contact area immediately after the droplet hits the surface. Moreover, this maximum pressure is larger than the dynamic pressure which indicates the effect of water-hammer-like pressure during the early stage of the droplet impact.

Micropillar type three-axis force sensor for measurement of cellular force

This paper reports a three-axis force sensor that can measure cellular forces in real time with high sensitivity. The sensor features a photoresist micropillar fabricated on a flexible cross-shaped Si structure. The three dimensional forces acting on the micropillar can be detected from the resistance changes of three piezoresistors designed on the Si structure. Due to the flexibility of the Si beams, a sensing resolution on the order of several nN was obtained for both shear forces and normal force. Moreover, in our sensor design, the sensing beams are covered by a photoresist cap that prevents cells from attaching to the piezoresistors while it maintains the space for the beams and the micropillar to deform. We confirmed that our sensor can detect the normal and shear forces acting on the micropillar caused by an osteosarcoma cell during its detachment from the surrounding walls.

Measurement of the pressure distribution during the onset of slip

We report on the direct measurement of the pressure distribution of the contact surface between an elastic block and a rigid plate during the onset of slip. The measurement was carried out by using an array of seven MEMS pressure sensors embedded inside the elastic block. The results showed that the external loading force in lateral direction caused the inhomogeneous change in pressure along the contact surface: the pressure in area close to the trailing edge decreased while that at area close to the leading edge increased. This result explains why the local slip always started to occur from the trailing edge where the pressure became small.

Piezoresistive cantilever integrated microfluidic channel for measuring cellular properties

We propose a microfluidic device that has piezoresistive cantilever on the bottom wall of its microchannel to measure cellular mechanical property. This sensor allows us to measure force applied to the wall of microfluidic channel directly with high sensitivity and high temporal resolution. Moreover, feedback from the sensor can be used to control liquid flow above the cantilever. We demonstrate that our device is able to detect the force acting on the channel bottom wall during the passing-through of mammalian cells by the piezoresistive cantilever.